The retrograde transport of horseradish peroxidase will be used to label neurons in mouse SmI cortex whose axons project to known regions of the brain. Cells labeled in this fashion are completely stained enabling synapses of their spines, dendritic shafts and local axon collaterals to be examined. Current efforts are directed at elucidating thalamocortical synaptic connections of neurons whose axons project from SmI cortex to ipsilateral MsI cortex.